1. The Field of the Invention
This invention relates to hot dip metallic coating of ferrous strand, and more particularly to the continuous hot dip coating of ferrous strand such as steel strip and wire with molten zinc or an alloy of zinc and other metals such as aluminum.
2. Description of the Prior Art
In the continuous hot dip metallic coating of ferrous strand including steel strip, sheet stock, and wire, hereinafter generally referred to as strip, the strip is treated in a furnace to provide a clean surface substantially free of oxide scale and other surface contamination so as to be readily wettable by the molten coating metal. The strip normally passes through an enclosed protective hood which extends from the treatment furnace and terminates in a snout having an open end projecting below the surface of the molten coating metal. The clean, treated strip passing through the hood is maintained at a temperature substantially equal to the temperature of the molten coating metal bath, and a reducing atmosphere such as a mixture of hydrogen and nitrogen is maintained in the hood to prevent the formation of oxides on the steel surface between the furnace and the coating bath.
When the coating metal is zinc, or an alloy comprising a substantial proportion of zinc, oxide vapors are evolved above the surface of the coating metal within the hood snout despite the reducing atmosphere maintained in the hood. Solid oxides of the coating metal tend to precipitate from the oxide vapor in the snout and the precipitated oxide may be deposited as a white powdery material on the surface of the coating bath or directly onto the surface of the hot strip entering the bath. The small particles of oxide adhering to the strip can result in a defective coating by preventing the coating material from adhering to the metal surface. For example, a particle of zinc oxide on the surface of a strip passing between the strip and the sink roll in the coating metal pot will tend to be crushed and prevent adherence of the coating metal over an area greater than the original size of the particle. Thus, it is desirable to remove the oxide containing vapors from the hood snout before it can precipitate in solid form.
The problem caused by coating metal oxides in hot dip coating steel strip with a zinc-aluminum alloy are discussed in U.S. Pat. No. 4,053,663. This patent also proposes to solve the problem by directing the reducing atmosphere gas consisting of a mixture of nitrogen and hydrogen gases into the hood so as to cause the gas to flow across the surface of the coating metal toward the steel strip from both sides, then upward along the strip. The reducing gas was preheated to about 1000.degree. F. before being introduced into the hood. Up to 24,000CFH were caused to sweep across the coating metal and flow countercurrent to the strip in the hood.
Applicant has discovered that directing large volumes of gas over the surface of the coating metal and against the surface of the strip can cause oxide particles to adhere to the surface to be coated. Further, the oxide vapors evolved in the hood of this prior art patent are retained in the hood and swept along the surface of the steel strip from the coating metal surface to the treatment furnace. However, in accordance with the present invention, the oxide vapors are removed from the protective hood before substantial quantities of the oxides precipitate as solids.
It is a primary object of the present invention to provide a method of and an apparatus for effectively and efficiently removing the coating metal oxide vapors evolved in the protective hood above the molten metal surface in a hot dip metal coating operation.
Another object of the invention is to provide an improved method and apparatus for withdrawing reducing atmosphere gas and coating metal oxide containing vapors from an outlet in the hood snout at a location near the surface of the coating metal, and to simultaneously introduce a nonoxidizing gas into the hood snout at a point to cause a flow of gas across the coating metal surface within the snout in the direction of the outlet to sweep coating metal oxide containing vapors from within the hood snout.
Another object of the invention is to provide an improved method of and apparatus for withdrawing coating metal oxide containing vapors and reducing atmosphere gas from the hood snout in a hot dip steel strand coating operation, removing the metal oxide from the vapors and reducing atmosphere gas removed from the snout, and reintroducing the oxide free gas into the hood snout.